Flash memory devices have developed into a popular source of non-volatile memory for a wide range of electronic applications. Flash memory devices typically use a one-transistor memory cell that allows for high memory densities, high reliability, and low power consumption. Common uses for flash memory include personal computers, flash drives, digital cameras, and cellular telephones. Program code and system data such as a basic input/output system (BIOS) are typically stored in flash memory devices for use in personal computer systems.
FIG. 1 shows a plot of VWL versus time of typical prior art programming and verification operations. The figure shows the series of incrementally increasing programming pulses 101 being applied to the target memory cell as the word line voltage VWL. The programming pulse increases a charge level on a floating gate of the target memory cell, thereby increasing the cell's threshold voltage Vt. After each programming pulse 101, a verify pulse 102 occurs at a Vvfy level to determine if the cell's threshold voltage has increased to the target programmed level.
After programming, the memory cell can experience multiple forms of charge loss. These include single bit charge loss, intrinsic charge loss, and quick charge loss.
Single bit charge loss is the result of a defective memory cell that exhibits electron leakage from the floating gate through the tunnel oxide to the active region. This leakage is typically due to oxide defects or trap assisted tunneling and results in inferior long-term data retention.
Intrinsic charge loss is a detrapping of electron traps near the tunnel oxide interface out to the channel region. Intrinsic charge loss can be accelerated with high temperature stress and occurs over a long period of time. The trapped charge initially causes the cell Vt to appear higher than the floating gate is programmed. The detrapping of these electrons long after programming then causes a onetime shift in the threshold voltage.
Quick charge loss is a detrapping of electron traps near the tunnel oxide interface out to the channel region and causes an immediate Vt shift after a programming pulse. When a cell passes the verify operation, the programmed threshold voltage appears to be higher due to the trapped charge in the tunnel oxide. When the cell is read after the program operation has been completed, the cell has a Vt that is lower than the Vt obtained during the program verify operation due to the charge in the tunnel oxide leaking out to the channel region. This can require an enlargement of the Vt distribution in order to accommodate all possible threshold voltages for a given state.
FIG. 2 shows the resulting Vt of the target cell being programmed. The top Vt plot 212, 216 is the maximum threshold voltage and the lower Vt plot 211, 214 is the minimum threshold voltage, as illustrated in FIG. 3. As the programming pulses 101 of FIG. 1 are applied to a target cell control gate, the Vt 211, 212 increases to approximately the Vt—vfy level. Once at this level, the target cell is verified and inhibited from further programming. The ideal Vt 213, 215 is shown staying level at Vt. However, the real Vt 214, 216 of the target cell begins to decrease almost immediately after the last programming pulse.
FIG. 3 illustrates a typical prior art Vt distribution of memory cells programmed in the manner shown in FIG. 1 to a target programmed state. In FIG. 3, the dotted line 300 represents the ideal distribution while the solid line 301 represents the real distribution. The lower end 305 of the ideal distribution 300 corresponds to memory cells having a Vt in accordance with plot 213 of FIG. 2 and the upper end 310 of the ideal distribution 300 corresponds to memory cells having a Vt in accordance with plot 215. Similarly, the lower end 306 of the real distribution 301 corresponds to memory cells having a Vt in accordance with plot 214 and the upper end 307 of the real distribution 301 corresponds to memory cells having a Vt in accordance with plot 216.
The cells at the lower end of the ideal distribution 300 are verified at the Vpgm—vfy voltage. After the programming operation and subsequent inhibition of those cells, the distribution shifts in the negative direction by an amount equal to VQCL and ends at the lower Vt 306. Such a shift in the distribution would necessitate an enlarged distribution that starts at the real lower Vt 306 and extends to the ideal upper Vt 310.
In a single level cell (SLC) memory device, a Vt distribution enlargement does not typically affect the reading of a programmed memory cell. However, in a multiple level cell (MLC) memory device, the state distributions are typically more closely spaced in order to fit all of the states within a low supply voltage range. Enlarging the Vt distributions in an MLC device can thus reduce the number of states that are programmable into the device. Additionally, the enlarged Vt distributions can overlap and result in errors in reading the different states.
For the reasons stated above, and for other reasons stated below that will become apparent to those skilled in the art upon reading and understanding the present specification, there is a need in the art to reduce the effects of charge loss in a memory device.